<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
body>div{
    width: 300px;
    background-color: rgba(0,0,0,0.3);
    padding: 10px;
    position: relative;
}
input{
    width: 260px;
    border: 0;
    padding: 10px 20px;
}
img{
    position: absolute;
    top: 14px;
    right: 30px;
}
p{
    font-size: 12px;
    color: red;
    margin: 0;
}
  </style>
</head>
<body>
<div>
  <input type="text" placeholder="请输入用户名">
  <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
  <p>用户名不能为空！</p>
</div>
</body>
</html>